The present invention relates to a transversal electric charge transfer filter with cut electrodes.
It will be recalled that a transversal cut electrode filter is generally formed by a semiconductor substrate covered by an insulating layer on which are alternated transfer electrodes and charge-storing electrodes, periodically receiving given potentials. These electrodes are disposed parallel to each other and transversely in relation to the charge transfer direction. The storage electrodes are cut transversely, generally in two parts, and the quantities of charges under the two parts of each storage electrode are read differentially so as to achieve weighting of the signal to be filtered.
The article by Richard D. Baertsch et al, entitled "The design and operation of practical charge--transfer transversal filters", and appearing in the IEEE review Transactions on Electron Devices, volume ED-23, number 2, of February 1976, pages 133 to 142, deals with transversal cut electrode filters and with the problems which they pose; among these problems, there may be mentioned:
the need to carry out a reading of the differential charges so as to obtain positive and negative weighting coefficients; this differential reading is conventionally achieved by means of an external differential amplifier, which has the disadvantage of being cumbersome, or by means of a differential amplifier integrated on the same substrate as the filter itself and formed essentially of MOS transistors, which has the disadvantages of a much greater consumption than that of charge-transfer devices and the non-linearities likely to be introduced by the MOS transistors;
the high common mode during elaboration of weighting coefficients close to zero, which tends to cause saturation of the differential amplifier and which in any case reduces its signal to noise ratio for the differential signal diminishes without the capacity of the storage electrodes diminishing;
the disturbance of the reading of the charges under the storage electrodes by control signals applied to the different electrodes;
the high number of control signals.